1. Field of the Invention
The present invention relates generally to power converters and more particularly, to improved control circuitry for allowing the interconnection and control of a plurality of modular power supplies used to drive a common load.
2. Brief Description of the Prior Art
Current mode controlled power supplies, sometimes referred to as power converters, are commonly used to provide power for numerous types of electronic apparatus. One such apparatus and other prior art, are disclosed and discussed in the U.S. Pat. No. 5,428,523, issued to John E. McDonnal on Jan. 27, 1995.
The McDonnal apparatus uses a variable reference and a comparator to detect sudden changes in the current sharing signal of a module in an n+N redundant system of DC--DC converter modules. These changes, once detected, will trigger a bi-directional switch to open, thereby disconnecting the faulty module from the common current sharing bus and preventing the remaining modules in the system from being adversely affected. In general, the variable reference is designed such that its output tracks the current sharing bus with a slight built-in offset. The offset allows for some variation and noise within the current sharing signal of the module without causing false fault detection. While the variable reference tracks slow variations in the common current sharing bus, its output is designed to be slew rate limited so that it can not track fast changes. Accordingly, if a fast change occurs, a fault will be detected and the module will be disconnected as desired.
Since the McDonnal invention detects faults solely by comparing the current sharing signal to a variable reference, it can not detect faults that cause variations in the current sharing signal bus that the variable reference can successfully track. For example, the prior art system will not detect the class of faults that cause slow moving or steady state changes in the current sharing signal that results in a slow moving or steady state error in the output voltage of the system.
To address this issue, one might suspect that the speed of the variable reference be made arbitrarily slow to detect any class of faults desired. However, because of reasons to be explained below, it is undesirable to have the reference speed set so slow that normal input and load transients cause the system to de-couple.
The startup assist circuit of the above-referenced prior art provides varying amounts of "assistance" to the starting module as its output voltage changes. The assistance is provided by lowering the voltage at the starting modules sense terminals to a level that is below its set point voltage. The amount of assistance provided is a function of the module's set point voltage, the module's current output voltage, and the system's sense bus voltage. With some modules whose output set-point is below the sense bus voltage, the startup assistance provided will diminish to zero before the output has reached the bus voltage. In these cases coupling of the starting module's current sharing signal to the current share bus will only occur if two conditions are met:
1. The current needed during startup (to charge internal and external capacitors) requires a current sharing signal that is higher than the variable reference derived from the common current sharing bus; and PA1 2. The coupling circuit responds quickly enough to couple the current share signal of the starting module to the common current share buss before it drops back down to its idle state.
If these two conditions are met, the unit will be coupled into the system. If they are not met, the starting module will idle with its output at some intermediate voltage below the common output bus voltage.
There are also some non-fault conditions or events that can cause a particular module's current sharing signal to become disconnected from the common current sharing bus. Some examples are high speed load transients, high speed transients on the common current sharing bus, and noise. When these conditions or events occur, some or all of the modules may be de-coupled from the common current sharing bus. Because of the variable assistance of the prior art startup assist circuit, a de-coupled module is not guaranteed to re-couple with the common current sharing bus unless either its output voltage set-point is higher than the sensed output voltage, or condition 2 above applies. If a module is not re-coupled it will idle with its output at some intermediate voltage below the common output bus voltage.
In addition, the startup assist circuits of the prior art can be subjected to relatively high levels of power dissipation during fault conditions in which the output of a module is shorted. These conditions require that the startup assist resistors either be oversized to handle the power dissipation during the fault or that protection circuits be added to limit the power dissipation in the resistors.
3. Objectives of the Present Invention
It is therefore an objective of this invention to improve the fault detection capability of the prior art decoupling circuits to detect faults that result in slow moving or steady state errors in the current sharing signal of the faulty module.
Another objective of this invention to improve the startup assist circuitry to provide constant amounts of "assistance" for all de-coupled modules for as long as they are de-coupled.
Still another objective of this invention to improve the startup assist circuitry by reducing the dissipated power during fault conditions thereby reducing the cost and size of the startup assist circuitry.